PRACTICAL DESIGN OF MICROWAVE DIODE FREQUENCY MULTIPLIERS Mihail Plamenov Tonev Technical University of Sofia, Bulgaria, Faculty of Tlecommunication, TU-Sofia, “Kl.Ohridsky” str. 8, 1000 Sofia, Bulgaria E-mail: [email protected] Abstract Phase noise is a critical performance parameter of frequency synthesizers for wireless communications. In phase modulated equip- ment the integrated phase noise in frequency synthesizer directly impacts to bit error rate of communication system. Design of fre- quency synthesizers with low phase noise is related to using low noise voltage controlled oscillators. Realizing such oscillators in microwave frequencies is hampered by low quality factor of tuning elements. A method to generate high frequency signal with low phase noise is to generate high quality lower frequency signal and perform frequency multiplication to deliver desired frequency. The purpose of this sheet is to give some theoretical insight for frequency multiplication and some practical guidelines for microwave diode multipliers design. 1. INTRODUCTION microwave frequencies may occurs problem with upconverting low frequency noise. The reason for The key element in designing microwave equip- this effect is that almost all transistors have high ment is availability of clean local oscillator. Clean gain at low frequencies. This problem is not pre- means a signal free from any sort of spurious fre- sented in diode frequency multipliers but they have quencies or parasitic modulation effects. In most worse efficiency. Diode multipliers are divided into cases spurious frequencies can be filtered by using two gropes – reactive multipliers and resistive multi- appropriate filter. The parasitic modulation with low pliers. Reactive multipliers can be realized by using frequency noises of active element, known as varactor or step recovery diodes. They have good phase noise can not be filtered. Unfortunately fre- efficiency but bandwidth in narrow. Good decision to quency multiplication process degrades this pa- realize microwave frequency multipliers with rela- rameter. The reason about this unfortunate charac- tively high bandwidth is to use resistive diode ap- teristic is that the frequency multiplier in fact is proach. This type of multipliers has bad efficiency but phase multiplier. So it multiplies phase deviations wide bandwidth and good noise performance. That’s as well as the frequency of input signal. The mini- why they are main topic in this article. mum carrier to noise degeneration, ∆CNR in deci- Depending on how diodes distort sinusoidal sig- bels, caused by frequency multiplier is: nal, multipliers are divided into two groups – odd order and even order. The schematics and output CNR20log N , waveforms of these multipliers are shown on fig. 1 and fig. 2. In odd order frequency multipliers the where N is multiplication factor spectrum of output signal contain only odd har- In spite of this fact, multiplying a very sable low- monic numbers and in even order multiplier is the frequency reference signal can still produce signals opposite. with better phase noise then producing them di- rectly in microwave frequencies range. Frequency multiplication is obtained by applying nonlinear distortion to sinusoidal signal. The spec- trum of distorted input signal contains a number of harmonics and suitable harmonic number can be Fig. 1. Even order frequency multiplier filtered and amplified to obtain output signal with desired frequency and amplitude. Frequency multi- Unwonted harmonics must be filtered and their pliers can be realized either with transistors or di- level has to meet requirements by standard for de- odes. Transistors provide better efficiency but in signed communication equipment. If unwonted har- CEMA’12 conference, Athens, Greece 81 monics are not suppressed enough, at the output of In general microwave resistive diode multipliers multiplier need to be placed more complicated band are realized with dual in package shotky diodes. In pass filter. Designing of such filter is difficult and this case the diodes have almost the same parame- sometimes impossible because it can not be manu- ters and asymmetry due to their difference can be factured within acceptable tolerances. That’s why neglected. Here more substantial parameter, which harmonic suppression is critical parameter for fre- influence the duty cycle is parasitic capacitances in quency multipliers. Since the bad efficiency can be diode package and asymmetry in layout due to compensated by amplifier with higher gain, the bad manufacturing tolerances. Most critical element unwonted harmonic suppression can make design from PCB layout in microwave frequencies is impossible. Improving this parameter is the main grounding holes. In fig. 3 is shown equivalent circuit topic of this paper. of two shotky diodes in package SOT23. This model was used for designing frequency multiplier x5. The input signal is in frequency range from 1.9 GHz to 2.3 GHz. The circuit was simulated with harmonica simulator, provided by ansoft. Fig. 2. Odd order frequency multiplier 2. UNWONTED HARMONIC SUPPRESSION IN MICROWAVE FREQUENCY MULTIPLIERS As was explained above, harmonic content at the output of multiplier depends on how diodes distort the input sinusoidal signal. The generation of odd order harmonics requires paralleling the diodes as shown in fig. 2. In this case the first diode con- ducts during the positive semi cycle while the sec- ond conducts during the other semi-cycle. If applied voltage is high enough, the result is a symmetrical distortion in waveform, resembling square wave- form. Theoretically if the duty cycle of this square waveform is 50%, its spectrum will contain only odd harmonics, and even harmonics suppression would Fig. 3. Nonlinear model for dual shotky diodes in SOT23 package be infinity. Unfortunately, in practice 50% duty cycle can’t be obtained because ideally symmetrical It wasn’t surprising, that simulator shows perfect structure is impossible to be manufactured. Theo- even harmonic suppression. Here asymmetry in retically the influence of duty cycle can be evalu- layout is not taken into account. To gain real insight ated be performing Fast Fourier Transforming to a how circuit works the simulated layout of frequency square wave signal and results of this research are multiplier was manufactured. Material used in de- presented in Table 1. In this case is performed mul- sign is Arlon25N with deictic permeability 3.38, tiplication x5 and suppression of forth and sixth substrate thickness 0.51mm and loss tangent harmonics is most important, because they are 0.0025. In fig.4 is shown measured test fixture. closest the desired harmonic. If we assume 20dB to be good even harmonic suppression, duty cycle of the output signal can be in range of 49.4 to 50.6%. Table 1. Evaluating influence of duty cycle to even harmonic suppression Duty cycle [%] Fourth harmonic suppression [dB] 50 infinity Fig. 4. Test fixture for microwave diode multiplier x5 49.5 22 To measure even harmonic suppression was 49 17 used input signal with frequency 2.1 GHz and po- 82 CEMA’12 conference, Athens, Greece wer level of 10dBm. Measured output spectrum is To decide this problem was manufactured an- shown in fig. 5. other test fixture with six grounding holes as shown in fig. 8. Multiplier x5 , 2 via, no input filter 15.00 10.00 5.00 0.00 ‐5.00 ‐10.00 m ‐15.00 [dB ‐20.00 P ‐25.00 ‐30.00 ‐35.00 ‐40.00 ‐45.00 Fig. 8. Redesigned multiplier with 6 grounding holes ‐50.00 0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 F [GHz] Measurement results show that, the redesigned Fig. 5. Measured output spectrum of diode frequency test fixture has almost the same harmonic suppres- multiplier x5 sion as the first one with solder drop, fig.6. This It’s obvious that fourth and fifth harmonic have decides problem with manufacturing automation but almost the same levels and we can’t speak for any 8dB harmonic suppression is still not enough. This even harmonic suppression. At fist sight the design frequency multiplier is suitable for use in VSAT is absolutely filed. To reduce the influence of down converters and VSAT standard require un- asymmetry in grounding holes, was put one big wonted harmonic suppression to be at least -60dBc. solder drop between grounded pins of diode, as If we assume that even harmonic suppression of shown in fig. 6. frequency multiplier is 8dB, to meet the standard we need filter which attenuate fourth and sixth har- monic at least 52dB. This requirement makes filter design very complicated. To improve even harmonic suppression of fre- quency multiplier was researched the influence of input signal. Most of the signal sources do not gen- erate clean sinusoidal signal. It always contains high order harmonics – second, third etc. The ques- tion is “How harmonic content of input signal affect Fig. 6. Diode frequency multiplier with added solder drop between grounded pins the even harmonic suppression”. To find answer of this question was made analogy with frequency up The result of this experiment is shown in fig. 7. converter. The first harmonic from the spectrum of Here we notice improvement in harmonic suppres- input signal acts as local oscillator frequency for up sion with about 8 dB. converter. Its power is high enough to change di- This decision is not good from manufacturing odes nonlinear transconductance. The second view point. This solder drop must be putted manu- harmonic has much lower power level than first ally and manufacturing process cannot be auto- harmonic. It can be represented as RF signal for mated. the assumed upconverter. Furthermore if higher order harmonics exists, they also will be mixed with Multiplier x5 , 2 via, no input filte, solder drop 15.00 the first harmonic and products of this mixing will 10.00 5.00 appear at the output of frequency multiplier. We 0.00 assume that their level is low enough and they can ‐5.00 ‐10.00 be neglected.